The invention generally relates to the field of synchronous power generators, such as those used in combination with gas turbines. Specifically, the invention relates to synchronous turbine generators having main power windings and auxiliary power windings where the auxiliary power winding is coupled to a variable frequency drive system.
Synchronous power generators are commonly used by power utilities to produce electrical energy. Generators generally have an electromagnetic rotor that is surrounded by a stationary stator having conductive windings. Rotating magnetic fields from the spinning rotor create electric current in the armature windings in a stationary stator that surrounds the rotor. The current from these windings is output as electrical power from the generator. The stator generally has two or three armature windings, each of which have an induced current. These currents are synchronous, but out-of-phase with each other. The generator produces two- or three-phase alternating current as electrical power usable by electric power utility companies.
Synchronous power generators are often driven by gas turbines. Gas turbines have a rotating drive shaft that is coupled to the drive shaft and rotor of the generator. When running, the gas turbine turns the drive shaft and rotor causes the power generator to produce electricity. In a gas turbine and generator combined power unit, the generator is commonly adapted to alternatively function as a starting motor for the gas turbine. To start the gas turbine, the generator may be temporarily operated as a motor that is powered from an auxiliary electrical power source. Once the generator-motor accelerates the rotational speed of the drive shaft sufficiently to start the gas turbine, the gas turbine is started. Once started, the gas turbine begins to output power to the driving the drive shaft and the generator, and the motor is switched back to operate as a generator.
A variable frequency power supply, e.g., a thyristor frequency convertor (TFC), drives the generator as a motor to start the gas turbine. The TFC may be referred to as a “static start” drive. The TFC applies alternating current to the stator windings to cause the generator rotor to turn which powers a drive shaft coupled to the gas turbine. The TFC gradually increases the frequency of the voltage applied to the stator to increase the rotational speed of the drive shaft. As the rotational speed of the rotor and drive shaft increases, the turbine is accelerated to its rated starting speed, where the turbine becomes self-sustaining and generates output power to drive the generator.
A conventional gas turbine frequency starting process accelerates the gas turbine up to a self-sustaining rotating speed by operating the generator as a motor. The generator-motor combination unit is driven by a static start drive (TFC) that is switched on and off by a synchronizer control circuit. In particular, the static start drive is switched on to drive the generator as a motor and then switched off as the gas turbine reaches its rated start speed. The rated start speed of the turbine is substantially slower than the synchronous speed at which the turbine generator operates during power generation. The TFC and exciter are conventionally turned off as the turbine accelerates from the rated start speed to the synchronous speed.
The exciter applies a low-level direct current (D.C.) to the windings of the generator rotor. During the static start process, the exciter is first switched on as the TFC drives the generator as a motor. The exciter is later switched off when the static start drive is switched off and as the gas turbine reaches its rated starting speed. The exciter is finally switched on again during a synchronization process when the electrical power output of the generator is synchronized with the desired load, e.g. an electrical power grid.
The conventional static start starting process generally has reliability problems due to increased technical requirements for switching on, off and then on again the excitation system and static start drive of turbine-generator combination unit. The conventional starting process also has difficulties with applying a turbine regulator to match the speed of the combination unit to the synchronous speed at which the generator may be connected to a balanced power load. Accordingly, there is a long felt need for a turbine-generator unit starting process that better handles accelerating the unit from the turbine rated start speed to the synchronous speed of a power load on the generator.